The purported head of al-Qaeda in Iraq has offered a reward for the murder of a Swedish cartoonist over his drawing depicting the Prophet Muhammad.

The $100,000 (£49,310) reward would be raised by 50% if Lars Vilks was "slaughtered like a lamb" said the audio message aired on the internet.

The speaker, said to be Abu Omar al-Baghdadi, threatened a new offensive during the holy month of Ramadan.

Last month's cartoon showed Prophet Muhammad's head on a dog's body.

Several Muslim countries protested.

It is fundamental for Western thinking to be able to express one's artistry without making exceptions for holiness

Lars VilksSwedish cartoonist

Last year there were riots over Danish cartoons of the Prophet Muhammad, first published in September 2005 by the newspaper Jyllands-Posten.

Muslims regard any visual representation of the Prophet as blasphemous. Many Muslims also regard the dog as an impure animal.

In other developments:

at least 10 people were killed and 15 injured when a suicide bomber blew himself up near a market in the district of Amil in Baghdad

Iraq's defence ministry said it shared a common vision with the US over when Iraqi forces may take over security responsibilities from US troops

Economic targets

The latest cartoon was published by the Nerikes Allehanda newspaper on 18 August.

Last week, Sweden's Prime Minister Fredrik Reinfeldt met ambassadors from 22 Muslim countries in an effort to defuse the row.

But Saturday's taped message said the militants were announcing a "call to shed the blood of the Lars who dared to insult our Prophet".

Last month some 200 Muslim protesters demanded an apology

"During this generous month we announce an award worth $100,000 to the person who kills this infidel criminal," the speaker said.

He also announced a $50,000 reward for the killing of the editor of the newspaper.

And he said the Swedish government ought to apologise - otherwise al-Qaeda in Iraq would target "their economy and giant companies such as Ericsson, Volvo, Ikea, Scania".

The cartoon's creator, Lars Vilks, told Reuters news agency he was not worried by a threat from people representing "a very small branch of our Muslims".

But he could not disregard it, either, and was in contact with the police.

"It is fundamental for Western thinking to be able to express one's artistry without making exceptions for holiness," he said.

Factional threats

The speaker in the 30-minute tape also said he was "honoured to announce at the beginning of Ramadan an offensive in the name of... the martyr of the Islamic nation Abu Musab al-Zarqawi" who was killed in a US air strike last year.

And days after saying they had killed a well-known Sunni cleric opposed to them, the speaker accused the main Sunni party, the Islamic Party in Iraq of co-operating with US-led forces.

Abdul Sattar Abu Risha was one of al-Qaeda's top targets

He also vowed to keep targeting members of the minority Yazidi community over their leaders' refusal to allow Yazidis to convert to Islam.

Meanwhile, in northern Iraq, a Sunni Arab tribal leader told the BBC that local groups had created a new alliance to fight al-Qaeda.

Fawwaz al-Jarba, who heads the Shammar tribe in the Mosul area, said local Sunni Arab tribes had joined Kurdish, Christian and Yazidi groups in a new front.

He said the new alliance would not work directly with the United States military, but only through the Iraqi government.

In July, the US announced that a top al-Qaeda militant arrested in Iraq had told interrogators that Iraq's supposed al-Qaeda kingpin, Omar al-Baghdadi, was only a front.